I need to acquire a general purpose RISC machine of some sort as a learning platform. My employer is a SPARC licensee but I don't actively work on those particular embedded systems. I do work on embedded systems with MIPS architecture CPUs but, as my main purpose is to learn-by-doing about RISC in general, I don't have a strong preference of MIPS over SPARC. So I have a few questions:

Which of those is more actively developed on in Gentoo?

I'd also like to acquire a used system on (for instance) eBay for a tolerable price. Suggestions would be appreciated.

Is there any other reason I should consider choosing one over the other?

- John_________________I can confirm that I have received between 0 and 999 National Security Letters.

sparc and mips are actively developed in Gentoo, but both could need a helping hand or two.

Are you looking for something enterprisey? Pre-owned UltraSPARC III/IV servers should become (relatively) cheap now that Oracle dropped Solaris 11 support for pre-T1 CPUs. These make great development boxes apart from the noise they produce.
MIPS has the advantage that you can get it in convenient form for carrying around with you, e.g. the Lemote Yeelong 10.1" netbook with 900 MHz Loongson-2F CPU.

Why do you want RISC, for programming on the hardware? While coding in MIPS assembly is no pleasure, SPARC assembly is definitely in DO NOT WANT territory.

No, not really enterprisey. More workstationey. I want to learn a RISC assembly language—more than academically, that is. MIPS probably makes more sense because the products I touch on a daily basis use SoCs with MIPS cores and, although it seems the industry is turning to ARM over the long term, I don't yet have a platform that I work on with an ARM core.

- John_________________I can confirm that I have received between 0 and 999 National Security Letters.

If you decide to go MIPS, get a box with the Chinese Loongson CPU. In Europe, you can get them starting from 215 EUR +VAT+shipping. http://www.tekmote.nl/
In case you want something smaller, many SOHO routers use some form of MIPS, e.g. the WZR-HP-G300NH (75 EUR) or the TL-WR1043ND (45 EUR). As long as OpenWRT runs, then Gentoo will too. Note that installing Gentoo on embedded devices usually requires careful preparation.

Old SGI MIPS hardware is also available on eBay, but I'd rather avoid that.

The story is that armin76 maintains SPARC almost exclusively, but he does that in addition to a huge number of other things, i.e., alpha, ia64, m68k, s390, and sh. He does a great job, but as you'd expect from not having a full team, things fall through the cracks. Gentoo/SPARC could definitely use your help.

As for MIPS, for some time it's been maintained almost exclusively by me. We have a relatively large team, but for a variety of reasons the members aren't active. I could use your help here too.

They have dual CPUs, either 1.28 GHz or 1.6 GHz. In fact, I have one that I'm not using (dual 1.28 GHz).

For MIPS hardware, as chithanh mentions, SGI hardware is available, but each model has unique drawbacks. The latest generations are fast, but unsupported. The early generations are totally supported, but horribly slow. In between -- a mixing.

I'd recommend a Loongson MIPS laptop. Loongson-2f hardware is available now, such as the Yeeloong. They're rather slow though, with a terrible graphics chipset.

Hopefully before too much longer the Loongson 3A laptop will be available. Loongson 3A is a quad-core CPU, so it's a massive improvement over 2f.